Head-Mounted Displays (HMDs), which include near eye displays in a form resembling conventional eyeglasses or sunglasses, are being developed for a range of diverse uses, including military, commercial, industrial, fire-fighting, and entertainment applications. For many of these applications, there is particular value in forming a virtual image that can be visually superimposed over the real-world image that lies in the field of view of the HMD user. Light guides incorporating various types of waveguides, relay image-bearing light to a viewer in a narrow space, acting as exit-pupil expanders for redirecting the virtual image to the viewer's pupil and enabling this superposition function.
In the conventional light guide, collimated angularly related light beams from an image source are coupled into the light guide substrate, generally referred to as a waveguide, by an input optical coupling such as an in-coupling diffraction grating, which can be formed on a surface of the substrate or buried within the substrate. Other types of diffractive optics could be used as input couplings, including diffractive structures formed of alternating materials of variable index such as holographic polymer dispersed liquid crystal (HPDLC) or volume holograms. The diffractive optics could also be formed as surface relief diffraction gratings. The collimated light beams can be directed out of the waveguide by a similar output optical coupling, which can also take the form of a diffractive optic. The collimated angularly related beams ejected from the waveguide overlap at an eye relief distance from the waveguide forming an exit pupil within which a virtual image generated by the image source can be viewed. The area of the exit pupil through which the virtual image can be viewed at the eye relief distance is referred to as an “eyebox.”
The output coupling can also be arranged for enlarging the exit pupil. For example, the collimated beams can be enlarged in one dimension by offsetting partially reflected portions of the collimated beams in a direction at which the collimated beams propagate along the output coupling or by ejecting collimated beams of different angles from different positions along the waveguide to more efficiently overlap the collimated beams at the eye relief distance from the waveguide.
A so-called “turning optic” located along the waveguide between the input coupling and the output coupling, can be used for expanding pupil size in a second dimension. The expansion can be effected by offsetting reflected portions of the collimated beam to enlarge a second dimension of the beams themselves or by directing the collimated beams to different areas of the output coupling so the collimated beams of different angles are ejected from different positions to more efficiently overlap within the eyebox. The turning optic can also take the form of a diffractive optic and, especially when located between the diffraction gratings of the input coupling and output coupling, can also be referred to as an intermediate grating.
Although conventional light guide mechanisms have provided a significant reduction in bulk, weight, and overall cost of display optics, there are still issues to resolve. Suitable separation of color channels is need in order to prevent cross-talk, in which color is processed and displayed from the wrong color channel. Cross-talk can lead to disparity between the color image data and the displayed color, and can also be a cause of objectionable color shifts, perceptible across the image field. Attempts to correct this problem have included stacking approaches in which multiple waveguides are stacked together with optional filters to prevent color from being directed to the wrong channel. Stacking, however, leads to thicker devices, adds weight, reduces brightness, and has not provided highly satisfactory results.
Thus, it can be appreciated that there is a need for improved designs that still provide the pupil expansion capabilities of the optical light guide, but allow these devices to be thinner and more lightweight, without compromising image quality and color balance.